diff --git a/src/PubSubClient.cpp b/src/PubSubClient.cpp
index 0dd85400..d77c14a6 100755
--- a/src/PubSubClient.cpp
+++ b/src/PubSubClient.cpp
@@ -474,6 +474,14 @@ bool PubSubClient::loop() {
             _client->stop();
             pingOutstanding = false;
             return false;
+        } else if (_bufferWritePos > 0) {
+            // There is still data in the buffer to be sent, so send it now instead of a ping
+            if (flushBuffer() == 0) {
+                _state = MQTT_CONNECTION_TIMEOUT;
+                _client->stop();
+                pingOutstanding = false;
+                return false;
+            }
         } else {
             this->buffer[0] = MQTTPINGREQ;
             this->buffer[1] = 0;
@@ -524,7 +532,6 @@ bool PubSubClient::publish(const char* topic, const uint8_t* payload, size_t ple
 bool PubSubClient::publish(const char* topic, const uint8_t* payload, size_t plength, uint8_t qos, bool retained) {
     if (beginPublish(topic, plength, qos, retained)) {
         size_t rc = write(payload, plength);
-        lastOutActivity = millis();
         return endPublish() && (rc == plength);
     }
     return false;
@@ -544,11 +551,7 @@ bool PubSubClient::publish_P(const char* topic, const uint8_t* payload, size_t p
 
 bool PubSubClient::publish_P(const char* topic, const uint8_t* payload, size_t plength, uint8_t qos, bool retained) {
     if (beginPublish(topic, plength, qos, retained)) {
-        size_t rc = 0;
-        for (size_t i = 0; i < plength; i++) {
-            rc += _client->write((uint8_t)pgm_read_byte_near(payload + i));
-        }
-        lastOutActivity = millis();
+        size_t rc = write_P(payload, plength);
         return endPublish() && (rc == plength);
     }
     return false;
@@ -588,6 +591,10 @@ bool PubSubClient::beginPublish(const char* topic, size_t plength, uint8_t qos,
 
 bool PubSubClient::endPublish() {
     if (connected()) {
+        if (_bufferWritePos > 0) {
+            // still data in the buffer to be sent
+            if (flushBuffer() == 0) return false;
+        }
         return true;
     }
     return false;
@@ -628,13 +635,21 @@ uint8_t PubSubClient::buildHeader(uint8_t header, uint8_t* buf, size_t length) {
 }
 
 size_t PubSubClient::write(uint8_t data) {
-    lastOutActivity = millis();
-    return _client->write(data);
+    return appendBuffer(data);
 }
 
 size_t PubSubClient::write(const uint8_t* buf, size_t size) {
-    lastOutActivity = millis();
-    return _client->write(buf, size);
+    for (size_t i = 0; i < size; i++) {
+        if (appendBuffer(buf[i]) == 0) return i;
+    }
+    return size;
+}
+
+size_t PubSubClient::write_P(const uint8_t* buf, size_t size) {
+    for (size_t i = 0; i < size; i++) {
+        if (appendBuffer((uint8_t)pgm_read_byte_near(buf + i)) == 0) return i;
+    }
+    return size;
 }
 
 /**
@@ -646,34 +661,48 @@ size_t PubSubClient::write(const uint8_t* buf, size_t size) {
  * @return True if successfully sent, otherwise false if buildHeader() failed or buf could not be written.
  */
 bool PubSubClient::write(uint8_t header, uint8_t* buf, size_t length) {
-    bool result = true;
-    size_t rc;
     uint8_t hdrLen = buildHeader(header, buf, length);
     if (hdrLen == 0) return false;  // exit here in case of header generation failure
 
+    return writeBuffer(MQTT_MAX_HEADER_SIZE - hdrLen, hdrLen + length);
+}
+
+/**
+ * @brief  Write the internal buffer to the client / MQTT broker.
+ *
+ * @param  pos Position in the buffer to start writing from.
+ * @param  size Number of bytes to write from the buffer.
+ * @return Number of bytes written to the client / MQTT broker (0 .. bufferSize). If 0 is returned a write error occurred or buffer index error.
+ */
+size_t PubSubClient::writeBuffer(size_t pos, size_t size) {
+    size_t rc = 0;
+    if (size > 0 && pos + size <= this->bufferSize) {
 #ifdef MQTT_MAX_TRANSFER_SIZE
-    uint8_t* writeBuf = buf + (MQTT_MAX_HEADER_SIZE - hdrLen);
-    size_t bytesRemaining = length + hdrLen;  // Match the length type
-    size_t bytesToWrite;
-    while ((bytesRemaining > 0) && result) {
-        yield();
-        bytesToWrite = (bytesRemaining > MQTT_MAX_TRANSFER_SIZE) ? MQTT_MAX_TRANSFER_SIZE : bytesRemaining;
-        rc = _client->write(writeBuf, bytesToWrite);
-        result = (rc == bytesToWrite);
-        bytesRemaining -= rc;
-        writeBuf += rc;
-        if (result) {
-            lastOutActivity = millis();
+        uint8_t* writeBuf = buffer + pos;
+        size_t bytesRemaining = size;
+        bool result = true;
+        while ((bytesRemaining > 0) && result) {
+            size_t bytesToWrite = (bytesRemaining > MQTT_MAX_TRANSFER_SIZE) ? MQTT_MAX_TRANSFER_SIZE : bytesRemaining;
+            size_t bytesWritten = _client->write(writeBuf, bytesToWrite);
+            result = (bytesWritten == bytesToWrite);
+            bytesRemaining -= bytesWritten;
+            writeBuf += bytesWritten;
+            if (result) {
+                lastOutActivity = millis();
+            }
+            yield();
         }
-    }
+        rc = result ? size : 0;  // if result is false indicate a write error
 #else
-    rc = _client->write(buf + (MQTT_MAX_HEADER_SIZE - hdrLen), length + hdrLen);
-    result = (rc == length + hdrLen);
-    if (result) {
-        lastOutActivity = millis();
-    }
+        rc = _client->write(buffer + pos, size);
+        if (rc == size) {
+            lastOutActivity = millis();
+        } else {
+            rc = 0;  // indicate a write error
+        }
 #endif
-    return result;
+    }
+    return rc;
 }
 
 /**
@@ -723,6 +752,35 @@ size_t PubSubClient::writeNextMsgId(uint8_t* buf, size_t pos, size_t size) {
     return pos;
 }
 
+/**
+ * @brief  Append a byte to the internal buffer. If the buffer is full it is flushed to the client / MQTT broker.
+ *
+ * @param  data Byte to append to the buffer.
+ * @return Number of bytes appended to the buffer (0 or 1). If 0 is returned a write error occurred.
+ */
+size_t PubSubClient::appendBuffer(uint8_t data) {
+    buffer[_bufferWritePos++] = data;
+    if (_bufferWritePos >= bufferSize) {
+        if (flushBuffer() == 0) return 0;
+    }
+    return 1;
+}
+
+/**
+ * @brief  Flush the internal buffer (bytes 0 .. _bufferWritePos) to the client / MQTT broker.
+ * This is used by endPublish() to flush data written by appendBuffer().
+ *
+ * @return Number of bytes written to the client / MQTT broker (0 .. bufferSize). If 0 is returned a write error occurred or the buffer was empty.
+ */
+size_t PubSubClient::flushBuffer() {
+    size_t rc = 0;
+    if (connected()) {
+        rc = writeBuffer(0, _bufferWritePos);
+    }
+    _bufferWritePos = 0;
+    return rc;
+}
+
 bool PubSubClient::subscribe(const char* topic) {
     return subscribe(topic, MQTT_QOS0);
 }
diff --git a/src/PubSubClient.h b/src/PubSubClient.h
index 01faf141..9b1857c0 100755
--- a/src/PubSubClient.h
+++ b/src/PubSubClient.h
@@ -178,6 +178,7 @@ class PubSubClient : public Print {
     Client* _client{};
     uint8_t* buffer{};
     size_t bufferSize{};
+    size_t _bufferWritePos{};
     unsigned long keepAliveMillis{};
     unsigned long socketTimeoutMillis{};
     uint16_t nextMsgId{};
@@ -197,9 +198,14 @@ class PubSubClient : public Print {
     bool readByte(uint8_t* result, size_t* pos);
     uint8_t buildHeader(uint8_t header, uint8_t* buf, size_t length);
     bool write(uint8_t header, uint8_t* buf, size_t length);
+    size_t writeBuffer(size_t pos, size_t size);
     size_t writeString(const char* string, uint8_t* buf, size_t pos, size_t size);
     size_t writeNextMsgId(uint8_t* buf, size_t pos, size_t size);
 
+    // Add to buffer and flush if full (only to be used with beginPublish/endPublish)
+    size_t appendBuffer(uint8_t data);
+    size_t flushBuffer();
+
    public:
     /**
      * @brief Creates an uninitialised client instance.
@@ -637,19 +643,33 @@ class PubSubClient : public Print {
 
     /**
      * @brief Writes a single byte as a component of a publish started with a call to beginPublish.
+     *        For performance reasons, this will be appended to the internal buffer,
+     *        which will be flushed when full or on a call to endPublish().
      * @param data A byte to write to the publish payload.
-     * @return The number of bytes written.
+     * @return The number of bytes written (0 or 1). If 0 is returned a write error occurred.
      */
     virtual size_t write(uint8_t data);
 
     /**
      * @brief Writes an array of bytes as a component of a publish started with a call to beginPublish.
+     *        For performance reasons, this will be appended to the internal buffer,
+     *        which will be flushed when full or on a call to endPublish().
      * @param buf The bytes to write.
      * @param size The length of the payload to be sent.
-     * @return The number of bytes written.
+     * @return The number of bytes written. If return value is != size a write error occurred.
      */
     virtual size_t write(const uint8_t* buf, size_t size);
 
+    /**
+     * @brief Writes an array of progmem bytes as a component of a publish started with a call to beginPublish.
+     *        For performance reasons, this will be appended to the internal buffer,
+     *        which will be flushed when full or on a call to endPublish().
+     * @param buf The bytes to write.
+     * @param size The length of the payload to be sent.
+     * @return The number of bytes written. If return value is != size a write error occurred.
+     */
+    size_t write_P(const uint8_t* buf, size_t size);
+
     /**
      * @brief Subscribes to messages published to the specified topic using QoS 0.
      * @param topic The topic to subscribe to.